Tubular furnaces are primarily used in oil refining and designed to burn fuel in a combustion chamber comprising a casing made of steel plates, the inner side of which is lined with refractory and heat insulating material, and to heat petroleum or other oil flowing in heating tubes (steel tubes) arranged within the combustion chamber by using generated heat.
Such a tubular furnace has an important problem of coking. The term "coking" means a phenomenon that a fluid to be heated is decomposed and altered into cokes, and taking steps to prevent coking is considered to be an important issue from the standpoints of design and operation in tubular furnaces which primarily handle hydrocarbon.
As steps to prevent coking, therefore, it has conventionally been practiced to select the value of heat flux such that a boundary layer temperature is held lower than a coking temperature, and to select a dimension of a tube diameter such that the flow velocity in the tube is held in an appropriate range. From this point, ordinary values of a heat flux and a flow velocity are specified in furnaces for heating residual oil that is highly likely to cause coking, such as furnaces heating raw material for atmospheric distillation apparatus or vacuum distillation apparatus, for example.
Meanwhile, from the viewpoint of energy conservation, a conventional tubular furnace is arranged as shown in FIG. 1, for example, such that a convection heat transfer section 102, in which a fluid to be heated is primarily heated by convection heat transfer, is provided in an upper part of a furnace 101, a radiant heat transfer section 103, in which the fluid is primarily heated by radiant heat transfer, is provided in a lower part of the furnace 101, and combustion gas generated by a burner combustion equipment 104 at the bottom section of the furnace 101 is exhausted through an exhauster 105 at the top of the furnace 101. A coil path in this furnace 101 is formed by connecting together the groups of heating tubes 106 arranged in the furnace into one unit of heating tube via U-shaped connecting tubes (not shown). The coil path has an inlet 107 located near the top of the furnace 101 in the convection heat transfer section 102 and an outlet 108 located near the bottom of the furnace 101 in the radiant heat transfer section 103. Therefore, the fluid to be heated, which is introduced into the heating tubes 106 from the inlet 107, is heated by the exhaust combustion gas at a relatively low temperature in the convection heat transfer section 102 and flowed in the downstream direction, and further heated by radiant heat of the combustion gas at a relatively high temperature in the radiant heat transfer section 103. Then, the fluid is drawn out from the outlet 108. In this case, since the boundary layer temperature of the fluid to be heated becomes maximum at near the outlet 108 of the coil path located in the radiant heat transfer section 103, the heat flux is set such that the boundary layer temperature of the fluid near the outlet 108 of the coil path is held lower than the coking temperature.
However, in the conventional tubular furnace, the inside of the furnace is heated by the burner 104 provided at the bottom section as one zone, with the result that a temperature in the furnace becomes lower as it proceeds towards the outlet of coil path located at the top end of furnace. Moreover, with the heat flux of the burner 104 being set such that the boundary layer temperature is held lower than the coking temperature at near the outlet 108 of the coil path where the boundary layer temperature becomes maximum, the heat flux is decreased down to an excessive small value as it proceeds towards the coil path inlet 107. In general, a usable maximum temperature of the furnace is defined by a wall thickness and a material of the heating tube 106, but in the present conventional case, such temperature is also determined in relation to the outlet 108 of the furnace 101 and thus, the heat flux near the outlet becomes an excessive small value as similar to the foregoing case for preventing coking. It is desired that heat flux in all areas of coil path should be increased up to a level close to a critical limit within which coking will not occur to raise heating efficiency. But, the heat flux in the conventional furnace 101 is smaller entirely except about the outlet 108 of the furnace 101, especially, the heat flux near the inlet 107 is a smaller value than desired so that heating efficiency is not so good and a big size furnace is required in order to increase the treating quantity and the refining quantity.
In addition, the convection heat transfer section 102 is provided in the upper part of the furnace 101, from which the combustion gas is exhausted, further to recover heat of the combustion gas becoming low temperature in the conventional furnace 101. Because of sulfur being contained in fuel, however, a tube wall temperature of heating tube 106 is required to be held higher than a acid dew point temperature from the standpoint of preventing low-temperature corrosion. This results in a problem that because of the combustion exhaust gas cannot being exhausted at a lower temperature, improvement of heat efficiency by recovering exhaust heat is not so sufficient and influences upon surrounding environment would be increased.